(a) Technical Field
The present invention relates to a thermal management system and method for a fuel cell vehicle. More particularly, the present invention relates to a thermal management system and method capable of performing selective de-mineralizing so as to improve a heat radiating performance in the thermal management system adopted in a fuel cell system.
(b) Background Art
A fuel cell system assembled in a fuel cell vehicle typically includes a fuel cell stack which generates electrical energy from an electrochemical reaction of reaction gas, a hydrogen supply apparatus which supplies hydrogen as a fuel to the fuel cell stack, an air supply apparatus which supplies air including oxygen to the fuel cell stack, and a thermal and water management system which dissipates the heat of the fuel cell stack to an environment outside of the fuel cell stack to optimally control an operating temperature and perform a water management function.
The fuel cell stack produces heat and water as reaction byproducts during an electrochemical reaction process of hydrogen and oxygen which are reaction gases and in order for the fuel cell stack to exhibit an optimal output performance, a temperature of the fuel cell stack needs to be managed at an optimum temperature at the time of starting or during operation. In particular, it is essential to use a thermal management system which rapidly increases the temperature of the fuel cell stack at the time of starting and maintains the temperature of the fuel cell stack at an optimum temperature during operation.
For example, a conventional thermal management system of the fuel cell vehicle is illustrated in FIG. 1. FIG. 1 is a schematic diagram illustrating a cooling water loop in a thermal management system of a fuel cell vehicle, in which the thermal management system of the fuel cell vehicle includes a radiator 2 which dissipates heat generated when the fuel cell stack 1 generates power, a cooling water circulating line 3 which is connected between the fuel cell stack 1 and the radiator 2 to be able to circulate cooling water therebetween, a bypass line 4 and a 3-way valve 5 which selectively bypass the cooling water to prevent the cooling water from passing through the radiator 2, a water pump 6 which pumps and circulates the cooling water, and a heater 7 which increases the temperature of the cooling water to warm the fuel cell stack.
In order to maintain electric conductivity of the cooling water at a predetermined level or less, a de-mineralizer (DMN) 9 which filters ions present in the cooling water may be disposed in a branch line 8 of the cooling water loop. The thermal management system dissipates the heat generated when the fuel cell stack generates power to the outside while circulating the cooling water along a path of radiator 2, to a 3-way valve 5, next to a water pump 6, then to a heater 7 and finally to a fuel cell stack 1.
In particular, the cooling water passing through the de-mineralizer 9 which is equipped in the branch line 8 of the cooling water loop again returns to the cooling water loop through a rear stage of the 3-way valve side as illustrated in FIG. 1. A connection structure between the 3-way valve and the branch line of the de-mineralizer is illustrated in more detail in FIG. 2.
As illustrated in FIG. 2, the 3-way valve 5 includes a first port 5a which is connected to a radiator side, a second port 5b which is connected to a bypass line, and a third port 5c which moves the cooling water passing through the two lines to the pump side.
Further, the branch line 8 of the de-mineralizer is connected to the third port side and due to a position of the branch line 8 which is disposed at an outlet of the third port, the flow rate of the de-mineralizer is always generated regardless of whether the 3-way valve is open or not.
Therefore, since the de-mineralizer loop is always opened, the high temperature cooling water always passes through the de-mineralizer loop and is unnecessarily subjected to de-mineralizing. As such, a service life of the de-mineralizer may be shortened.
Further, since the cooling water of about 10% of the entire cooling flow rate continuously flows through the de-mineralizer loop, the cooling flow rate is lost and the heat radiating performance is reduced.
Meanwhile, since a polymer electrolyte fuel cell (PEFMC) equipped in the vehicle is often operated at low temperatures, a radiator having a significant heat radiating area is required, but in a hot season, a heat radiating amount from the radiator may be less than a heating value of the fuel cell stack. Therefore, as illustrated in FIG. 3, when the temperature of cooling water at an outlet of the fuel cell stack is increased and thus reaches a set temperature, a fuel cell control unit (FCU) limits a current output of the fuel cell stack to protect the fuel cell stack to prevent the temperature of the cooling water from increasing beyond than the set temperature. This is referred to as a high temperature current limitation.
When the rapid acceleration and high output operation of the vehicle is extended over a pro-longed period of time (for example, driving on a highway or driving on an uphill road) or the flow rate of the cooling water is insufficient during a summer season, the cooling water reaches a higher temperature and thus a high temperature current limitation frequently occurs. As such, the output from the fuel cell stack is insufficient even though a driver presses an accelerator pedal during this limited current period.
Since there is a need to increase the insufficient heat radiating amount so as to prevent the high temperature current limitation from frequently occurring, a method for additionally increasing heat radiating areas of a radiator may be considered as an optional solution to this problem. However, the size of the radiator is limited by the vehicle layout configuration and thus a larger radiator is not desirable.
Further, heat radiating performance may be maximized by using the high performance/high flow rate pump. However, again there is a drawback to this. In particular, the high pressure of the cooling water generated at the time of the high flow rate operation of the pump may exceed an internal pressure level of the fuel cell stack. When this occurs, water leakage due to failure of the structure of the fuel cell stack may occur, and therefore this solution has its limitations as well.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.